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The Waikato Regional Flood
Event of 9-20 July 1998

The Australasian Journal of Disaster
and Trauma Studies
Volume : 1998-2


The Waikato Regional Flood
Event of 9-20 July 1998


AJ Munro, Flood Duty Officer, Environment Waikato, PO Box 4010, Hamilton East, New Zealand. Email: AdamM@wairc.govt.nz

Keywords: Waikato River, flood warning, flood management, water level, rainfall

A J Munro

Flood Duty Officer
Environment Waikato
PO Box 4010
Hamilton East
New Zealand


Abstract

This report gives a brief synopsis of the recent flood event on the Lower Waikato River, located approximately 50 km south of Auckland, New Zealand. Extremely high river levels were experienced on the Waikato and Waipa Rivers between 9-20 July due to widespread, and in places record rainfall. As a result, urban centres including Hamilton, Ngaruawahia, Huntly, and Mercer were inundated. Farms adjacent to the Waipa River, Whangamarino Wetland, and properties fringing Lakes' Taupo and Waikare were also affected. This event came off the back of a previous deluge that occurred only a week before. These back to back deluges (along with already saturated ground conditions) and successive frontal bands meant that rivers were unable to cope with the copious amounts of water entering the catchment. Flood warnings and management systems generally worked very well, and the Lower Waikato Waipa Control Scheme performed as per design.


The Waikato Regional Flood
Event of 9-20 July 1998


Background

The Waikato River is New Zealand's longest river (425 km), forming the centrepiece of the Waikato Region. It begins its long journey on the northeastern slopes of Mt Ruapehu (2797 metres), flowing into Lake Taupo. From the lake, the river cuts its way through the volcanic deposits of the central plateau, across which are eight hydro-electric dams, and onto the flat often swampy lowlands between Cambridge and Mercer. The river finally discharges into the Tasman Sea at Port Waikato, approximately 30 km south of Auckland. Overall, it drains water from 18,650 km2 or 12% of the area of the North Island.


Introduction

Very high river levels were experienced over the whole region during 9-20 July due to widespread, and in places record, rainfall. This report gives a brief overview of the flood event from the main catchments affected. Due to the size and nature of this event, a direct comparison is made where possible with the February 1958 Flood Event using both historical rainfall and river flow data (Tables 1 & 2). Information contained in this report is provisional, as further analysis and investigations are being undertaken.


Rainfall

Heavy rainfall was experienced throughout the region between 8-16 July 1998 with the heaviest falls being recorded in the Taupo, Rangitoto, Coromandel/Kaimai, Hapuakohe and Hunua Ranges. This was the second deluge, the other occurring less than a week earlier. These back to back deluges saturated the ground to the extent that river levels were unable to return to normal between the events. The upper catchment rivers therefore reacted very quickly upon the arrival of this second deluge.

The design rainfall frequency for the Lower Waikato Waipa Control Scheme is 200-250mm over three days.

The 7-day total rainfall from 8-15 July for the relevant rainfall recorders are presented in Table 1. Most of the rain fell 9-11 July, with Ngaroma (near Te Kuiti) receiving its July normal in less than two days. The 7-day figures are compared with rainfall in a 'normal' July.

Table 1 : Rainfall figures
Rainfall Site Catchment 8-15 July 1998, mm Normal July, mm % Above Normal Feb 1958, mm
Waikato/Waipa Catchments
Pukekawa Lower Waikato 165 150 10 -
Mangatangi Mangatangi 316 207 53 -
Maungakawa Mangawara/Piako 202 150 35 325?
Hamilton Waikato 191 123 55 324
Otewa Waipa 196 155 26 -
Ngaroma Waipa 361 223 62 340
Te Kuiti Mangaokewa 260 170 53 -
Hauraki/Coromandel Catchments
Smiths Kauaeranga 165 130 27 -
Pinnacles Kauaeranga 460 435 6 -
Golden Cross Ohinemuri 273 296 -8 -
Maukoro Piako 172 120 43 -
Waharoa Waitoa 138 128 8 -
Wharekawa Waihou 275 250 10 -
Te Aroha Waihou 150 166 -11 -
Kaimai Waihou 370 175 111 -
Taupo/Awakino Catchments
Wharekiri Mangaokewa/Waipa 250 197 27 -
Waitanguru Awakino/Mokau 286 304 -6 -
Turangi Tongariro 195 150 30 507

Waikato Map

Figure 1 : Map of the Waikato Region, showing the major rivers affected in this flood event.

River Flows

Recorded peak flows are presented in Table 2 below.

Table 1 : River flows (cumecs)

Location Peak Flow July 1998 and typical values (cumecs) Peak Date July 1998 Comparison cumecs
Taupo/Awakino Rivers
Poutu Intake (Tongariro) 660 - 9th 572 on 2/7/98
Tauranga-Taupo 218 13 10th 204 on 2/7/98
Awakino @ SH3 282 - 9th -
Waipa/Waikato Rivers
Mangaokewa @ Te Kuiti 122 11 9th 87 on 2/7/98
Waipa @ Otewa recorder damaged durning flood  
Waipa @ Otorohanga 418 52 10th 625 in 1958
Waipa @ Whatawhata 776 165 13th 1130 in 1958
Waikato @ Hamilton 807 306 15th 905 in 1958
Waikato @ Ngaruawahia 1491 456 12th 1482 in 1958
Waikato @ Huntly 1490 490 15th 1540 in 1958
Waikato @ Rangiriri 1490 507 15th -
Waikato @ Mercer 1575 545 16th 1260 in 1958
Hauraki Basin/Coromandel Rivers
Waihou @ Okauia 180 33 11th -
Waihou @ Te Aroha 190 53 16th -
Waihou @ Puke Bridge (tidal) - 15th -
Waitoa @ Waharoa 25 3 12th -
Waihou @ Mellon Rd 68 11 12th -
Piako @ Kiwitahi 72 4 11th -
Piako @ P-T Road 168 17 12th -
Piako @ Maukoro Landing (tidal) - 13th -
Kauaeranga @ Smiths 353 11 26th -
Ohinemuri @ Karangahake 295 21 15th -

Lake Taupo Levels and Gate Settings

Directly prior to the second event (9 July), Lake Taupo was at a level of 356.94 metres, still 0.31 metres below the maximum winter control level. The control gates were set to minimum through the initial stages to minimise flooding downstream of Karapiro. The initial push of water through the hydro dams was to get rid of as much water as possible through the already swollen Hydro lakes before the Waipa peak arrived at Ngaruawahia.

The Taupo control gates were again set to minimum on 15 July to reduce lower Waikato River levels. Between 16-17 July, river levels generally declined, with Lake Taupo swelling to its highest level in 40 years (357.49 m) due to continued high tributary input. In fact, it has been calculated that the total inflow for Lake Taupo for July 98 is the biggest monthly inflow since records began in 1905.

The Tongariro Power Development scheme (diversion of foreign waters) was shut down when Lake Taupo reached its winter maximum control level of 357.25 metres.

On 17 July, Lake Taupo control gates and Karapiro Dam discharges were increased to reduce Lake Taupo levels. The control gates were maintained at their maximum setting until the level of lake fell significantly below the winter maximum control level of 357.25 m (this was reached on Monday 3 August, more than two weeks after the gates were fully opened).

The main effects from these high lake levels were:

Lake Taupo erosion

Figure 2: Shoreline erosion at Lake Taupo (due to high lake levels)

Note: maximum lake levels for planned storage are 357.14 m and 357.25 m, for summer (January-March) and winter (April-December) respectively. The maximum level is lower in summer because cyclonic rainfall is more likely then.

Karapiro Flows

Environment Waikato and ECNZ worked around the clock to ensure that the Karapiro Hydro Dam was used to the best advantage possible (i.e. to minimise flooding downstream). The cascade effect and uncontrolled tributary inflows through the already full dams, made this process difficult. It became increasingly apparent that to effectively manage flood flows through the lower Waikato system, the flood managers needed a constant flow from Karapiro. Once the Taupo Control Gates were opened to maximum, a steady flow through the dams was finally achieved.

It has become apparent that the tributary inflows into the Waikato system between Karapiro and Mercer were significant (0.4 cumecs / km2 between Karapiro and Hamilton, as opposed to 0.12 cumecs / km2 between the dams).

It is apparent that the volume of water entering the Waikato system from minor tributaries between Karapiro and Mercer played a relatively major role in contributing to the flood levels experienced on the lower Waikato.

The peak discharge from Karapiro was 550 cumecs on 12/7/98. At Hamilton, the flow had increased to 750 cumecs due to another 200 cumecs entering the system between the two locations.

Without the effect of the hydro dams and the option to use Lake Taupo as a flood storage area, peak flood levels at Hamilton, Huntly, and Mercer would have been 1 metre, 0.5 metres, and 0.1 metres higher respectively.

Lake Waikare/Whangamarino Wetland Gate Settings

Both the Lake Waikare and Whangamarino Wetland Control Gates were closed on 10 July when the Waikato River level exceeded the water level of the wetland (both gates are either closed or opened simultaneously). Between 11-20 July, Lake Waikare steadily increased from 5.60 metres to peak at 6.29 metres (highest level since 1958) due mainly to the high inflows from the Matahuru Stream which continued at higher than normal levels for most of this period (due to further rainfall).

Many farms that fringe the lake were inevitably flooded as the local stopbanks (i.e. private) were overtopped. Likewise, some farms fringing the Whangamarino wetland were also inundated due to the swollen Maramarua and Whangamarino Rivers which continued to infill the wetland above private stopbank levels while the control gate remained closed.

Since the establishment of the Lake Waikare gate, the lake level can now be artificially controlled between 5.50-5.65 m. In pre-gate days, the average level of the lake hovered around 6.5 m (0.21 m higher than the flood peak in July 1998). The highest lake level on record is 8.38 m (1958). The design flood level of the scheme (when lake floodwaters actually flow over farmland and into the wetland) is 7.37 m. In this event, approximately 1.7 km2 of land was flooded by Lake Waikare.

The Whangamarino Wetland area swelled from its normal 17 km2 to 67 km2 in this event as opposed to 126 km2 in the 1958 flood. Without the presence of the gates, the wetland level would have been equal to the Waikato River level of 6.11 m or an extra 73 km2 of land underwater, potentially totaling 140 km2).

Flood Warnings and Alerts

The flood alert levels in the Taupo, Waipa, and Waikato River systems were quickly raised from 3 to 5 from the evening of 9 July when it became obvious that extremely high river flows were being generated in the upper catchments, and heavy rain was continuing to fall.

The flood alert levels for the Hauraki/Coromandel Rivers reached 4 in some areas.

The Flood Operations Centre was activated on Thursday 9 July at 11:00 pm. From that point on (until Monday 20 July), river levels and rainfall patterns were monitored by staff 24 hours per day. The telemetry system switched from normal day/night mode into alert mode meaning that sites were being polled (updated) every hour, instead of the usual 3 hours.

Environment Waikato flood duty staff kept in constant contact with district councils, ECNZ and residents living on or near the Waikato/Waipa Rivers, advising them of expected peak levels and times. Predicting the arrival of the peaks became a difficult exercise with the onset of more heavy rain, and the already high river levels from the previous event.

Regular contact was also made with the Paeroa Flood Response Officers in regard to the Piako River flood situation.

Flood warning recipients were notified by phone, fax, email, and/or paging. The 0832 Infolines were also operational during the event, and staff encouraged customers (where relevant) to use it as much as possible. Telecom has since advised that the Waipa/Waikato Infoline had a 335% increase in the number of calls between 1-20 July, taking pressure off key staff in the flood control centre.

Media Coverage

Media coverage of the event was extensive, shifting from a flood warning perspective in the initial stages, through to the effects of the flood on farms as far as recovery and compensation claims were concerned towards the end. In general, the link between the flood duty staff and the media worked extremely well. While the main media were radio and newspapers, some staff were interviewed by both TV 1 and TV 3 reporters. A compilation of TV news clippings relating to the flood event has been produced for future reference.

Flood Management Procedures

This flood event was managed with the assistance of the following documents:

Due to the complexities of this event, decisions at times were made with limited information available. Circumstances also changed very quickly, and were unpredictable.

The guidelines outlined in the above reports have been tailored specifically for the whole Waikato River system (i.e. including Lake Taupo, Waipa River, and other tributaries and designated flood storage areas). These are derived from many years of historical data, knowledge, and experience by the various scheme managers.


Flood Effects:

Taupo/Waikato/Waipa Rivers
The peak flow at Tauranga-Taupo (estimated to have a probability of occurrence in any one year of 5% (20 year flow) inundated SH 1 and the road was closed for some days.

The peak flow in the Awakino River Taupo (estimated to have a probability of occurrence in any one year of 20% (5 year flow) inundated SH 3, and the road was closed due to flooding and slips.

The peak flows in the upper Waipa were the highest recorded since 1958. The peak flows at Otewa and TeKuiti were estimated to have a probability of occurrence in any one year of 1% (100 year flow), and a peak flow at Otorohanga of about 2% (50 year flow), and at Whatawhata approximately a 1.5% probability of occurrence (80 year flow).

The peak flows in the lower Waikato system from Ngaruawahia to Mercer hovered between a 2% (50 year flow) and a 1% (100 year flow) probability of occurrence, which were in places higher than the 1958 peak flow. The peak flow at Mercer reached a level of 6.11 metres, inundating unprotected areas of the floodplain in this locality, including the Morrison Road basin and the Mercer riverbanks which are located outside the scheme area. The Rangiriri Spillway operated as designed for the first time in 28 years (Figure 3).

SH1 flooding

Figure 3 : Rangiriri Spillway allowing water to flow over State Highway 1 as per design. The road remained open throughout the event.

The nature of the second deluge was to produce high flows in all catchment areas (i.e. upper and lower) due to record rainfall that was fairly widespread. The flood wave increased in severity as it moved downstream due to the heavy input from the middle and lower Waikato tributaries. This was paramount in the lower Waikato as the peak flow from the Waikato Hydro system passed through Ngaruawahia before the Waipa peak flow arrived.

Over time, controlling the timing of the peaks proved extremely difficult due to continued rainfall, which either re-elevated river levels or attenuated the peak flow.

This meant that the Waikato peak flows through Hamilton, and the Waipa peak flows through Whatawhata were sustained at a very high level for some time.

The normal time of travel for flows between Otorohanga and Whatawhata is about 16 hours, but this peak took about 90 hours to reach Whatawhata.

Hauraki/Coromandel Rivers
The peak flow in the upper Waitoa river system equated to a 5% (20 year flow) probability of occurrence in any one year.

The Piako Ponding Zones were used for the first time as per the Piako River Scheme design, with significant flooding occurring in these zones.

Provisional Flood Damage - Rural
It is estimated that 1000-1500 hectares of farmland were flooded, predominantly in the lower Waikato area. Information provided from Federated Farmers indicate that about 9 farms were completely under water, with another 30-40 partially affected.

Provisional Flood Damage - Residential
The following residential areas were reported to have suffered some form of flood damage:

It has been estimated that the costs for the Waikato Region in regard to state highway damage will lie somewhere between $7-9 million. This includes costs of about $5 million for the large slip on SH 3 near Mohoenui. The total cost is expected to top $17 million, including damage to the protection scheme, infrastructure, farms, and property.


Review Programme (Where to from here)

Full Review
Flood Event
Flood Management
Scheme Management

Debriefings
Internal
Key Agencies
Community

Reporting

Auditing

Scheme Repairs

Longer Term (Improving our Performance)

Addressing identified issues

Review of the the Flood Rules and Flood Warning Manual

Scheme Protection Standards

Hazard Management
land-use planning
mapping of floodplain areas

Establishing Emergency Management Groups


Summary and Conclusions

The main causal mechanisms for this flood event were:

Flood flows in the Tauranga-Taupo, Tongariro, Waipa, and Waikato River systems in this event have been regarded (at some sites) as being larger than the 1958 flood, but not quite rivalling the 1907 event. Hence, the Alert Level assigned to these systems reached 5 (major flooding, minor roads flooded, and major flows in river system).

Flood warnings and management systems generally worked very well, and the Lower Waikato Waipa Control Scheme operated better than design in many places. The general reaction from the community, both from the urban and rural sector, has been extremely positive.

It should be noted that the majority of the previous major flood events that are commonly referred to, occurred during the summer period (i.e. cyclone originated), whereas this event has been out of the norm occurring during the winter months.

MetService have since indicated that a very warm north Tasman Sea provided the energy for the successive depressions that formed during July, bringing with them the active rainfall bands that resulted in this flood.


Copyright

A J Munro © 1998. The author assign to the Australasian Journal of Disaster and Trauma Studies at Massey University a non-exclusive licence to use this document for personal use and in courses of instruction provided that the article is used in full and this copyright statement is reproduced. The authors also grant a non-exclusive licence to Massey University to publish this document in full on the World Wide Web and for the document to be published on mirrors on the World Wide Web. Any other usage is prohibited without the express permission of the author.


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